Pathfinder's rule system is based off of D&D's 3.5 system. There's a contingent of D&D players who prefer 3.5 over 4th edition. 4th edition is very WoW healer/DPS/Tank spec which doesn't jive with some gamers, so a lot of them tried out Pathfinder. Paizo, the company behind Pathfinder, spent a lot of time testing and cleaning up some of the balance issues with 3.5 and just ran with it. They also provide a lot of quality content for running games which is very appealing to gamers.
Different strokes for different folks really. It actually shouldn't feel that much different from what you've been playing. You should probably double check with who's running the games to figure out if you need to come to the event with a character sheet pre-prepared or if you can come in completely blind. Because while I say that the two game systems "feel" the same, they certainly use different rule systems which you might need to be familiar with before the game begins.
